First focal length and second focal length of a lens will be the same only when

General lens equation is μ3V−μ1u=(μ2−μ1R1−μ2−μ3R2)For first focal length V=a and u=−f1∴ 1f1=(μ2−μ1)/μ1R1−(μ2−μ3)μ1R2For second focal length, u=α and V=+f2∴ 1f2=(μ2−μ1)/μ3R1−(μ2−μ3)/μ3R2Obviously when μ1=μ3f1 will be equal to f2